1. The Field of the Invention
The present invention relates to semiconductor optical amplifiers. More particularly, the present invention relates to semiconductor optical amplifiers and super luminance edge emitting light emitting diodes with a broadened gain spectrum where the gain of optical signals of varying wavelengths is balanced across a spectrum of wavelengths.
2. Background and Relevant Art
Optical communication systems have several advantages over other types of telecommunications networks. Optical fibers are typically made from insulative materials and are therefore less susceptible to interference from electromagnetic sources. Optical fibers also have higher bandwidth capability. In addition, optical fibers are both smaller and lighter than metal cables.
As optical signals are transmitted through the optical fibers of the communication network, the optical signals gradually become weaker over distance. Thus, the optical signals are typically refreshed or strengthened before the signals become too weak to detect. Before the advent of optical amplifiers, regenerators were used to refresh or strengthen the weakened optical signals. Regenerators convert the optical signal to an electrical signal, clean the electric signal, and convert the electrical signal back to an optical signal for continued transmission in the optical communication network.
Optical amplifiers, on the other hand, are superior to regenerators because optical amplifiers can amplify light signals of multiple wavelengths simultaneously while regenerators can only amplify one channel or a single wavelength. One type of optical amplifier is a semiconductor optical amplifier (SOA). At a basic level, an SOA includes multiple layers of compound semiconductor materials that are grown on a semiconductor substrate. An SOA usually includes an active layer and two cladding layers. The active layer provides optical gain to a light signal and the cladding layers together with the active or core layer, construct a optical waveguide.
The facets of the SOA are formatted by cleaving the semiconductor wafer along the crystal plane such that a mirror is formed. An antireflective (AR) coating is often applied to at least one facet in order to decrease the facet reflection. When an optical signal is injected at the input facet of the SOA, the light is amplified in the active layer by the gain of the SOA.
In some amplifiers, the active layer includes a pair of compound semiconductor material layers to confine the electrons inside the active layer. An extra pair of cladding layers is often needed to confine the optical mode. These extra layers of the SOA structure are a separate confinement structure that confine the optical modes. Thus, the electrons and the optical modes are confined separately by different layers.
Another problem associated with quantum wells is that the gain of optical signals varies according to the wavelengths of the optical signals being amplified. This problem is caused, for example, by the type of material included in the quantum wells of the active region. The thickness of the quantum wells also has an impact on the gain of the optical signal that is partly dependent on the wavelength of the incident optical signal.
In other words, semiconductor optical amplifiers suffer from the inability to provide the same or similar gain to optical signals across a spectrum of wavelengths. This is problematic for various reasons. For example, designing and engineering optical networks becomes more difficult as specific wavelengths of light must be considered instead of a spectrum of wavelengths.
These and other problems are overcome by the present invention which is directed to semiconductor optical amplifiers that generate essentially the same gain for optical signals of different wavelengths across a spectrum of wavelengths. The gain of optical signals within spectrum of wavelengths is flattened. The present invention is also directed to edge emitting light emitting diodes (edge emitting LEDs) whose structure is similar to semiconductor optical amplifiers.
The gain of a semiconductor optical amplifier can be flattened or made more constant by adding a gain balancing layer to the structure of a semiconductor optical amplifier or an edge emitting LED. The gain balancing layer effectively repositions an optical signal within an active region of the semiconductor optical amplifier such that the optical confinement factor of thin quantum wells is increased. By increasing the confinement factor of some quantum wells or of the active region as a whole, the gain of different wavelengths can be balanced.
One example of an active region of a semiconductor optical amplifier includes quantum wells where the quantum wells have different thicknesses with respect to the other quantum wells. In some instances, the quantum wells may be compressively strained quantum wells and/or tensile strained quantum wells. Alternatively, the quantum wells are neither compressively strained or tensile strained. After the active region has been formed and the cladding layers have also been formed around the active region, the gain balancing layer is added or formed to the structure of the semiconductor optical amplifier.
The gain balancing layer changes the overall refractive index such that the optical signal is repositioned within the active region. The gain balancing layer also has the effect of increasing the optical confinement of thinner quantum wells. With a gain balancing layer of appropriate thickness, the gain of the semiconductor optical amplifier can be balanced or flattened across a spectrum of wavelengths.
The gain balancing layer changes the refractive index profile of the semiconductor optical amplifier in the vertical direction such that the optical mode (or optical spot) is repositioned within the active regions. The spot of the optical signal propagating through the active layer can be adjusted in the vertical direction by changing a thickness of the gain balancing layer. By gain of the semiconductor optical amplifier can be flattened across a spectrum of wavelengths by adjusting the thickness of the gain balancing layer.
The proper thickness of the gain balancing layer can be determined by etching the gain balancing layer to different thicknesses. The gain of optical signals with different wavelengths are then measured to determine which thickness of the gain balancing layer enables the semiconductor optical amplifier to balance the gain across a spectrum of wavelengths. In this manner, the semiconductor optical amplifier has a gain that is balanced or flattened across wavelengths within the spectrum of wavelengths amplified by the semiconductor amplifier.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.